With the advent of interest in foam/skin insulated telephone wire, it has been learned that satisfactory results are difficult or impossible to attain by conventional co-extrusion systems. Such conventional equipment feeds the conductor to a tip and a primary melt is discharged along the outside of the tip and into contact with the conductor before the conductor and adhered melt passes through a first extrusion die. The outside of this first extrusion die is tapered to provide another tip surface along which a secondary melt is discharged into contact with the primary melt layer as the conductor and its adhered coating advance through the second tip. The conductor with the co-extruded layers then passes through a secondary extrusion die. In order to apply a foam/skin insulation over the conductor, the first extrusion die applies a cellular polyethylene layer, usually high density polyethylene, and the second extrusion die applies a solid polyethylene skin (high density) over the cellular insulation. The principal difficulties encountered with conventional co-extrusion systems relate to a lack of smoothness of the insulation; disruption of the cellular structure of the primary layer; and lack of dielectric strength. These problems are magnified as the melt index of the compound used for the outer skin is lowered.
This invention overcomes these difficulties by using only one tip and one extrusion die; and by applying the two layers of insulating material successively with a melt-flow separator between the supplies of insulation as they approach the end of the tip through which the conductor passes. With this invention, the primary and secondary insulations are sized simultaneously and no cellular or surface disruption occurs. Relative wall thickness of the primary and secondary insulations are controlled by adjusting the relative speed with which the different insulations are supplied to the extruder die.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.